1. Field of the Invention
The present invention relates to a low pressure CVD (chemical vapor deposition) system which is used to form a film of e.g. silicon nitride on semiconductor wafers.
2. Description of the Related Art
As such a low pressure CVD system, there has been used a system as shown in FIG. 6 (see JP-A-93166741 corresponding to U.S. Pat. No. 5,316,472). In the low pressure CVD system shown, a dual tube 13 is constituted by an outer tube 11 and an inner tube 12. A base portion 14 is arranged under the outer tube 11 and the inner tube 12 to support the tubes 11 and 12. The base portion 14 is provided with a gas feed pipe 15 leading to a space in the inner tube 12, and an exhaust pipe 16 leading to a space between the inner tube 12 and the outer tube 11.
The base portion 14 has a lower surface formed with an opening, and a lid 17 which is vertically moved by a lift 18 is arranged to open and shut the opening. The lid 17 has a wafer boat 20 placed thereon through a pedestal 19, and the wafer boat 20 supports numbers of semiconductor wafers W. When the lid 17 is raised by the lift 18 the wafer boat 20 and the wafers W placed on the lid 17 through the pedestal 19 are loaded into the dual tube 13 and the lid 17 seals the opening of the base portion 14. When the lid 17 is lowered by the lift 18, the opening of the base portion 14 is opened, and the wafer boat 20 and the wafers W which have been placed on the lid 17 through the pedestal 19 are unloaded. A cylindrical furnace 21 with a heater is arranged along an outer periphery of the dual tube 13.
In the conventional low pressure CVD system, quartz glass has been used as the material for the outer tube 11. The reason is that it is easy to obtain high-purity quartz glass, and that use of quartz glass is preferable to prevent contamination of the semiconductor wafers W. Since quartz glass has sufficient heat-resistance and a significantly low thermal expansion coefficient for the outer tube of a low pressure CVD system, no fracture due to thermal stress has not occurred even if a temperature difference is created between a portion of the outer tube which is arranged in the furnace 21 and a portion of the outer tube arranged below the furnace 21. Because quartz glass has low thermal conductivity the temperature of a lower end of the outer tube is hardly raised, and it is easy to provide hermetic sealing.
When the conventional low pressure CVD system is used to deposit a film on the semiconductor wafers W by CVD technique, CVD films are deposited on an inner surface of the outer tube 11 and on the inner tube 12 as well. Since the CVD films on the outer tube and the inner tube peel off easily to produce particles having created a problem in that the semiconductor wafers W are contaminated. As a result, it has been necessary to frequently carry out operations wherein the outer tube 11 and the inner tube 12 are cleaned to remove the CVD films deposited on the tubes.